1. Field of the Invention
The present application relates generally to processes for providing a protective treatment to metal thin films. In some embodiments, thin films used in metal gate and metal electrode applications in metal oxide semiconductor field effect transistors (MOSFETs), such as n-channel MOSFETs (NMOS) are treated either during or after deposition in order to prevent or reduce the effects of oxidation.
2. Description of the Related Art
Oxidation of a metal thin film can easily occur during many steps in processing, such as by exposure to atmospheric water or oxygen. In a multi-step fabrication process oxidation may occur between the deposition of each thin film, such as when transferring a wafer or substrate between deposition modules. Oxidation poses a problem in that it can affect the workfunction of a given thin film or an entire stack. And oxidation in one thin film may lead to oxidation of the interface between that film and a second film or even oxidation of the second film itself if the oxygen is able to diffuse through the first film to the second film.
For example, in a typical fabrication process of a MOSFET, oxidation of the etch-stop layer can easily occur after formation of a PMOS stack and before formation of an NMOS stack. Oxidation of the etch-stop layer can affect the workfunction of the subsequently formed NMOS stack, as it may lead to a shift in the workfunction, for example, from n-type to p-type. Other layers deposited during formation of a gate stack can also be exposed to oxygen, for example between deposition of each of the various thin films.
Referring to FIG. 1, a typical NMOS stack 100 is illustrated. The stack 100 includes a dielectric layer 102, a first metal nitride layer 104, a metal carbide layer 106—in which the interface 108 between the first metal nitride layer 104 and the metal carbide layer 106 includes oxidized portions represented by the presence of oxygen (“O”) atoms—a second metal nitride layer 110, and a metal layer 112. The presence of oxygen at the interface 108 between the first metal nitride layer 104 and the metal carbide layer 106 can undesirably shift the workfunction of the stack 100 from n-type to p-type.
Oxidation of the various layers can occur in a variety of ways during formation of the stack; however, it is common for the first metal nitride layer 104 to have already been oxidized prior to the deposition of the metal carbide layer 106. Even if the metal carbide layer 106 is able to be deposited without the presence of oxygen so as to achieve a relatively pure layer of a metal carbide, oxygen present in the first metal nitride layer 104 is capable of diffusing up into the metal carbide layer 106. Oxygen in the metal carbide layer 106 and particularly at the interface 108 can undesirably shift the work function of the overall stack 100.